Preparation of mesoporous ZnAl2O4 nanoflakes by ion exchange from a Na-dawsonite parent material in the presence of an ionic liquid

Herein, mesoporous ZnAl2O4 spinel nanoflakes were prepared by an ion-exchange method from a Na-dawsonite parent material in the presence of an ionic liquid, 1-butyl-2,3-dimethylimidazolium chloride ([bdmim][Cl]), followed by calcination at 700 °C for 2 h. The as-obtained products were characterized by several techniques such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The ZnAl2O4 nanoflakes with the thickness of ∼20 nm were composed of numerous nanoparticles, which resulted in a high specific surface area of 245 m2 g−1. The formation mechanism of the ZnAl2O4 nanoflakes was comprehensively investigated, and the results showed that a 2D growth process of the Zn6Al2(OH)16(CO3)·4H2O crystallites with the assistance of [bdmim][Cl] was the key for the induction of ZnAl2O4 nanoflakes. Moreover, mesopores were formed between adjacent nanoparticles due to the release of CO2 and H2O molecules from Zn6Al2(OH)16(CO3)·4H2O during the calcination process.


Introduction
Zinc aluminate (ZnAl 2 O 4 ) spinel is a well-known wide band gap semiconductor (E g ¼ 3.8 eV) 1 ceramic material with optomechanical property, which has been extensively studied as a catalyst and catalyst support, 2-4 transparent conductor, 5 dielectric material, 6 optical material 7 and sensor 8 due to its high thermal stability, low surface acidity and high mechanical resistance. 9 Several methods, for example, the solid statereaction or ceramic method, 10 wet chemical routes, 11,12 the sol-gel method, 13,14 the hydrothermal method, 15 the solvothermal method, 16 the plasma method, 17 combustion in an aqueous solution 18 and molten salt synthesis 19 etc., have been applied for the preparation of ZnAl 2 O 4 spinel. However, to date, it is still a challenge to synthesize 2D or 3D ZnAl 2 O 4 spinel nanostructures via a solution route. Although hierarchical ZnO-Al 2 O 3 microspheres have been reported, 20,21 to date, the pure phase of 2D ZnAl 2 O 4 nanoakes has not been prepared via a facile ion-exchange method.
Dawsonite (denoted by Na-Dw) is a mineralogical nomenclature that specically refers to the naturally formed sodium hydroxyalumino-carbonate, NaAl(CO 3 )(OH) 2 . 22 The Na-Dw compounds have been applied as ingredients in antacids, 23 stabilizers in polymers, 24,25 dry extinguishers in fuel leak res, 26 additives in synthetic fertilizers 27 and precursors for pure alumina. [28][29][30] Recently, the advantages of ionic liquids (ILs) have been gradually discovered in the synthetic processes of inorganic nanomaterials. [31][32][33][34][35][36][37] Particularly, ILs have received signicant attention as templates in the synthesis of numerous functional materials. Our research group has successfully synthesized various functional nanostructures using ionic liquids as so templates, reactants or precursors. [38][39][40][41][42] Herein, we present mesoporous ZnAl 2 O 4 nanoakes prepared by the ion-exchange method from a Na-Dw parent material in the presence of an ionic liquid, [bdmim][Cl], followed by calcination at 700 C for 2 h. The product exhibits a thin ake-like morphology, composed of numerous nanoparticles, and has a high specic surface area, 245 m 2 g À1 . To the best of our knowledge, this is the rst time that ZnAl 2 O 4 spinel nanoakes have been prepared by the ion-exchange method from a Na-Dw parent material.

Synthesis of the Na-Dw parent material
The Na-Dw parent material was prepared by the coprecipitation method reported in the literature. 43 Typically, 2 mmol of AlCl 3 $6H 2 O was dissolved in 15 mL of deionized water under constant stirring, and then, 10 mmol of NaHCO 3 was slowly added to the solution at room temperature. The obtained gel was hydrothermally treated at 120 C for 12 h in a 20 mL Teonlined stainless steel autoclave. Aer the reaction was completed, the autoclave was naturally cooled down to room temperature. Then, the slurry was centrifuged and washed several times with distilled water and ethanol. The white solid residues were dried at 80 C for 2 h, and thus, the Na-Dw parent material was obtained.

Synthesis of the ZnAl 2 O 4 nanoakes
In a typical preparation of ZnAl 2 O 4 , 0.6 mmol (0.1 g) of Na-Dw and 1 mmol of the ionic liquid [bdmim][Cl] were added to 10 mL of 0.03 M Zn(NO 3 ) 2 aqueous solution, and then, the mixture was slowly stirred and maintained at 50 C for 10 h. The as-obtained white precipitate was centrifuged, washed several times with distilled water and ethanol, dried at 80 C for 2 h, and nally calcined at 700 C for 2 h to obtain the ZnAl 2 O 4 nanoakes.

Characterizations
The products were characterized by XRD, FTIR spectroscopy, SEM, TEM and EDX. XRD measurements were performed using the Rigaku D/max 2500 diffractometer with Cu Ka radiation (l ¼ 0.154056 nm) at V ¼ 40 kV and I ¼ 150 mA, and the scanning speed was 8 min À1 . TGA measurements were performed using the DuPont Instruments 951 Thermogravimetric analyzer from room temperature to 725 C in owing nitrogen gas at the heating rate of 5 C min À1 . The FTIR spectroscopy of the sample was conducted at room temperature with a KBr pellet using the VECTOR-22 (Bruker) spectrometer in the range from 400 to 4000 cm À1 . The morphologies of the samples were studied by eld emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F). The TEM and HR-TEM images and EDX spectra were obtained using the Tecnai G2 20S-Twin transmission electron microscope operating at the accelerating voltage of 120 kV. The specic surface areas (S BET ) of the samples were calculated by following the multipoint Brunauer-Emmett-Teller (BET) procedure using the Quantachrome Nova 2000e sorption analyzer. The pore diameter and the pore size distribution were determined by the Barrett-Joyner-Halenda (BJH) method. Fig. 1a shows the XRD pattern of the Na-Dw parent material prepared at 120 C in 12 h. All detectable peaks in this pattern can be assigned by their peak positions to orthorhombic NaAl(CO 3 )(OH) 2 (JCPDS: . No evidence could be found for the existence of other impurities in the product aer washing. The XRD pattern of the precursor prepared by ion exchange using the Zn(NO 3 ) 2 solution is shown in Fig. 1b, which clearly shows two types of characteristic diffraction peaks that can be indexed to monoclinic Al 2 O 3 $3H 2 O (JCPDS: 01-0259) and hexagonal Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O (JCPDS: 38-0486). The XRD patterns of the samples obtained aer calcination of the precursor at 500 and 700 C for 2 h ( Fig. 1c and d), respectively, present the characteristic diffraction peaks of the cubic phase ZnAl 2 O 4 spinel (JCPDS: 05-0669); they indicate that the mixed crystalline phases of the precursor have been converted to the pure ZnAl 2 O 4 spinel crystalline phase upon heat treatment, and higher calcination temperature is benecial for the enhancement of crystallinity. As is well-known, the average size of the nanocrystal can be calculated via the Scherrer formula: 44

Results and discussions
where D hkl is the particle size perpendicular to the normal line of the (hkl) plane, K is a constant (it is 0.9), hkl is the full width at half-maximum of the (hkl) diffraction peak, q hkl is the diffraction angle, and l is the wavelength of X-ray. The average size of the ZnAl 2 O 4 spinel nanocrystal calculated from the strongest diffraction peak (311) shown in Fig. 1d is about 4.7 nm. The lattice parameter of the crystal was calculated based on the Xray diffraction pattern using the following equation, 44 where a is the lattice parameter, d hkl is the interplanar spacing corresponding to the Miller indices, and h, k, l are the miller indices. The calculated lattice parameter of the spinel ZnAl 2 O 4 product ( Fig. 1d) is 8.195Å, which is very close to the theoretical value of gahnite (8.0848Å). The abovementioned results are similar to those reported in previous studies. 18,45 The thermal stability of the precursor prepared by ion exchange at 50 C in 10 h was investigated by TGA and DTG. As shown in Fig. 2, the precursor exhibits the total weight loss of about 36.6%. Based on previous studies, [46][47][48][49] we believe that the thermal decomposition process includes ve steps as follows: (1) a 7.2% weight loss from 30 to 110 C due to the removal of physically adsorbed water and part of crystal water from Al 2 -O 3 $3H 2 O (eqn (2)), (2) a 2.7% weight loss from 110 to 150 C due to the phase transition from Al 2 O 3 $3H 2 O to AlOOH (eqn (2)), (3) a 7.4% weight loss from 150 to 230 C, assigned to the removal of structural interlayer water of the Zn 6 Al 2 (OH) 16 (3)), (4) a 16.1% weight loss from 230 to 500 C, attributed to the removal of residual crystal water, CO 2 molecules and part of hydroxyl groups from the crystals of AlOOH and Zn 6 Al 2 (OH) 16 (CO 3 ) (eqn (5) and (6)), and (5) a 3.2% weight loss above 500 C due to the removal of residual hydroxyl groups. Based on the TGA results, we proposed the formation process of the ZnAl 2 O 4 nanoakes as follows: Zn 6 Al 2 (OH) 16 ZnO The theoretical total weight loss during the thermaldecomposition process is 32.8%, which is consistent with the TGA result.
The FTIR spectrum of the product obtained aer calcination of the precursor at 700 C for 2 h is shown in Fig. S1(a) in the ESI. † It displays a strong band around 3460 cm À1 , which is attributed to the vibration of the OH group bonded to the surface. The band at 1610 cm À1 is associated with the vibration of Al-OH, characteristic of ZnAl 2 O 4 , and the weak peak at 1398 cm À1 is due to the HOH vibration of water. The wide band from 797 to 497 cm À1 is related to the inorganic network, including the Zn-O bending vibrations, Al-O stretching vibrations and Al-O-Zn stretching vibrations. 45,50,51 As shown in Fig. S1(b) in the ESI, † the local composition EDX spectrum reveals that the stoichiometric atom concentration ratio is Zn : Al : O z 15.6 : 29.3 : 55.1% z 1 : 2 : 4, conrming that the as-obtained product is ZnAl 2 O 4 . Moreover, a Cu signal located at 8.1 eV was revealed, which originated from a copper grid used in the HR-TEM measurement.
The morphologies of the products were characterized by FE-SEM. The FE-SEM image of Na-Dw is shown in Fig. S2 in the ESI, † which exhibits a nanorod shape. However, well-developed nanoakes were obtained aer Zn 2+ ion exchange reactions in the presence of the ionic liquid (Fig. 3a). To clarify the effect of the ionic liquid [bdmim][Cl] on the morphology of the product, a control experiment was carried out in the absence of the ionic liquid, and other reaction conditions were kept constant. The FE-SEM image of the as-obtained product is shown in Fig. S3 in the ESI, † which displays an irregular shape with few nanoakes. These results imply that in the present reaction system, the ionic liquid has an important effect on the morphology of the product; moreover, ion exchange occurs between Zn 2+ ions and Na-Dw molecules dissolved in solution, and then, new structures can be formed via recrystallization of the product molecules. There is no signicant role of the ionic liquid in the ion exchange process, whereas in the crystal growth process, the ionic liquid plays a crucial role. Moreover, the abovementioned results reveal that the ILs can have an important effect on the morphologies of the inorganic materials at the very low temperature of about 50 C; in an IL-templated system, the nanostructures of inorganic materials are generated by a hydrogen bonding-co-p-p stacking mechanism, as discussed in previous studies; 40-42 the morphology of the sample aer   calcination at 700 C for 2 h is well-preserved, and the sample still possesses a nanoake shape (Fig. 3b). Fig. 4 shows the TEM images of the product obtained by calcination at 700 C for 2 h, which display a nanoake-like morphology, and each nanoake is composed of numerous nanoparticles with the diameters of about 20 nm. There are many mesopores between adjacent nanoparticles (Fig. 4b). The typical lattice spacing was determined to be 0.29 nm, corresponding to the (220) lattice plane of ZnAl 2 O 4 (inset in Fig. 4b).
The effect of the mole ratios of Zn 2+ : Na-Dw on the crystal phase of the precursors was investigated on the basis of control experiments. As shown in Fig. 5, two crystal phases of Al 2 O 3 -$3H 2 O and Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O co-existed in the precursor when the mole ratio was 1 : 2. As the mole ratio of Zn 2+ : Na-Dw was increased, the concentration of the Zn 6 Al 2 (OH) 16 (CO 3 )$ 4H 2 O phase gradually increased; when the mole ratio of Zn 2+ : Na-Dw reached 3 : 1, pure phase of the Zn 6 Al 2 (OH) 16 (-CO 3 )$4H 2 O crystal was obtained. To further clarify the effect of the Zn 2+ : Na-Dw mole ratio on the product structure, XRD analysis of the products obtained by calcination of the precursors at 700 C for 2 h was carried out, as shown in Fig. S4 in the ESI. † When the mole ratio was 1 : 2, pure ZnAl 2 O 4 crystals could be obtained. In other cases, however, ZnO and ZnAl 2 O 4 coexisted in the products. Moreover, as the mole ratio increased, the ZnO phase became the main crystal phase of the product.
These results reveal that the mole ratio of Zn 2+ : Na-Dw significantly inuences the product composition, and the optimal mole ratio is 1 : 2 to obtain the pure phase of ZnAl 2 O 4 nano-akes. Fig. S5 in the ESI † shows the morphologies of the precursors obtained using Zn 2+ : Na-Dw at different mole ratios. It can be observed that there are no signicant changes in the morphology of the precursors with a change in the mole ratios; this indicates that the nanoakes are formed from Zn 6 Al 2 (-OH) 16 (CO 3 )$4H 2 O rather than from Al 2 O 3 $3H 2 O or ZnO.
As is well-known, well-developed alumina nanostructures, such as AlOOH or Al 2 O 3 , can only be obtained at higher reaction temperatures by hydrothermal synthesis 52 or solvothermal synthesis. 53 Thus, in the present reaction system, it is impossible for the alumina crystals to develop well because of the low reaction temperature of 50 C. Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O, known as a layered double hydroxide (LDHs) or hydrotalcite-like compound, exists either as a natural mineral or a synthesized material. It has a sandwich structure composed of a cation (Zn 2+ and Al 3+ ) layer (octahedron) and an anion (CO 3 2À ) interlayer, both of which are quite tunable (Scheme S1 in the ESI †). 54,55 Considering its structural characteristics, in the present reaction system, the crystal growth of Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O is a dominant factor in the formation of well-developed 2D ake-like nanostructures via the recrystallization process of the mixed crystals obtained aer ion-exchange reactions. According to the abovementioned discussions, we believe that the ionic liquid molecules adsorbed on the surface of the Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O crystallites play an important role as templates or structural indicators; however, they also adsorb on the surface of the Al 2 O 3 $3H 2 O crystallites. Since the pH value of the present reaction system is about 7 and the PZCs of Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O and Al 2 O 3 -$3H 2 O crystals are about 11.5 and 9.7, 56,57 respectively, the surfaces of the abovementioned two kinds of crystallites are positively charged. Therefore, the ionic liquid molecules adsorb on the surfaces of the crystals through an anionic dominant model. 39 The schematic of adsorption is shown in Scheme 1.
To investigate the formation process of the ake-like ZnAl 2 O 4 nanostructures, we carried out analogous experiments for different reaction durations, as shown in Fig. 6.    6a shows that irregular particles are rst formed aer reaction for 1 h at 50 C. The morphology of these particles is entirely different from that of the Na-Dw parent material; this indicates that the ion-exchange process is accompanied by the dissolution of precursor molecules rather than simple in situ ion exchange. When the reaction time was extended to 2 h, some nanoake-like structures appeared (Fig. 6b). Large-scale underdeveloped nanoakes were formed aer a 4 h reaction (Fig. 6c), and well-developed nanoake-like structures could be obtained aer an 8 h reaction (Fig. 6d). Moreover, there was no signicant change in the morphology aer a 10 h reaction.
Based on the abovementioned experimental results, a possible formation process of the ake-like ZnAl 2 O 4 was proposed. In the rst stage, irregular particles were formed via dissolution of the Na-Dw parent material, which underwent ion exchange with Zn 2+ ions and reprecipitated in sequence, as shown in Scheme 2. In the subsequent stage, the dissolutionrecrystallization process dominated, and the [bdmim][Cl] molecules adsorbed on the surface of the irregular particles as a so template to control the direction of the crystal growth, as illustrated in Scheme 1. Herein, the Cl À ions from [bdmim]Cl preferentially adsorbed on the building blocks of hydrotalcitelike Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O due to the formation of hydrogen bonds between Cl À ions and Zn 6 Al 2 (OH) 16 (CO 3 )$4H 2 O molecules; then, the [bdmim] + ions also adsorbed on the abovementioned building blocks due to electrostatic interactions. As previously reported, the [bdmim] + ions have a great tendency to self-assemble into ordered structures that are stabilized by additional p-p interactions along the aligned hydrogen bonds. 40 In the last stage, an Ostwald ripening process dominates, and consequently, well-developed 2D ake-like nanostructures are obtained. Based on the abovementioned discussions, we proposed the formation mechanism of the mesoporous ZnAl 2 O 4 spinel nanoakes, as illustrated in Scheme 3.
To investigate the specic surface area and porous nature of the ZnAl 2 O 4 spinel nanoakes, Brunauer-Emmett-Teller (BET) gas-sorption measurements were carried out. The nitrogen adsorption/desorption isotherm obtained for the product shows signicant hysteresis at the relative pressure P/P 0 of above 0.71 (Fig. 7). Moreover, the BET specic surface area of the product was calculated, which was about 245 m 2 g À1 , higher than the previous research results: 183.5 m 2 g À1 , 18 182.8 m 2 g À1 (ref. 45) and 147 m 2 g À1 . 58 The Barrett-Joyner-Halenda (BJH) calculations for the pore-size distribution, derived from the desorption data, reveal a narrow pore distribution with one apex centered at 14.5 nm (inset of Fig. 7), indicating that the asobtained ZnAl 2 O 4 spinel product has mesopores. These mesopores presumably arise from the spaces between adjacent  nanoparticles formed during the calcination process due to the release of CO 2 and H 2 O molecules from Zn 6 Al 2 (OH) 16

Conclusions
In summary, the well-developed mesoporous ZnAl 2 O 4 spinel nanoakes were successfully prepared by the ion-exchange method using an aqueous solution of Zn(NO 3 ) 2 and Na-Dw parent materials in the presence of an ionic liquid, [bdmim] [Cl], at 50 C, followed by calcination at 700 C for 2 h. The formation mechanism of the ZnAl 2 O 4 nanoakes was explored on the basis of control experiments and structure analyses. The results demonstrate that [bdmim][Cl] plays a crucial role in the formation of the ake-like morphology at the abovementioned low temperature. The optimal mole ratio of Zn 2+ : Na-Dw is 1 : 2 to obtain the ZnAl 2 O 4 spinel nanoakes. The BET-specic surface area of the mesoporous ZnAl 2 O 4 nanoakes, constructed by numerous nanoparticles, is as high as 245 m 2 g À1 . Since Na-Dw is a cheap natural mineral, the synthesis of ZnAl 2 O 4 spinel nanostructures at low temperatures using Na-Dw as a parent material can be applied as an economical and signicant industrial method. The mesoporous ZnAl 2 O 4 spinel nanoakes are expected to be used in some applications such as in catalysts and catalyst supports.

Conflicts of interest
There are no conicts to declare.